绕线式异步电动机转子电路串电阻的调速方法是属于改变（）调速。
A．磁极对数
B．转差率
C．电源频率
D．磁极的磁通
参考答案：B
I have no idea Jim could get so much money in such a short time.
A．what
B．how
C．why
D．when
